Merge "Add documentation on mkdir about seclabel" am: 715ecbfb23

Original change: https://android-review.googlesource.com/c/platform/system/core/+/2110586

Change-Id: I4eca56a15db5ef54eaa7ed92cdc83622fb4d7d54
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>